Multipurpose heat treatable aluminum alloys and related processes and uses

ABSTRACT

This application discloses an aluminum alloy, processes for preparing the aluminum alloy, processes for fabricating metal parts, such as automotive panels, comprising the aluminum alloy, and the automotive parts fabricated from the aluminum alloy.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional PatentApplication No. 62/078,027, filed Nov. 11, 2014, which is incorporatedby reference herein in its entirety.

FIELD OF THE INVENTION

The present invention relates to the fields of material science,material chemistry, metallurgy, aluminum alloys, aluminum fabrication,and related fields.

BACKGROUND

Aluminum alloys used for various applications must achieve certainproperties. For instance, aluminum alloys are used for fabrication ofinner and outer panels of transportation machinery. Aluminum alloys areuseful for this application due to a combination of their light weight,which leads to increased fuel efficiency, strength, and otherproperties. Among other things, the aluminum alloys used for fabricationof inner and outer panels of transportation machinery should possessgood formability, paint or other finish quality, dent resistance andimmunity to natural aging. It is also desirable for the alloys used inthe fabrication of transportation machinery to be recyclable. New andimproved metal alloys with desirable characteristics suitable forfabrication of transportation machinery panels can expand the range ofalloys available for these applications, lower the material costs,increase the aluminum recycling rates, decrease the capacity limits onthe production of such alloys, and decrease the environmental impact ofaluminum production and use.

SUMMARY

The terms “invention,” “the invention,” “this invention” and “thepresent invention” used herein are intended to refer broadly to all ofthe subject matter of this patent application and the claims below.Statements containing these terms should be understood not to limit thesubject matter described herein or to limit the meaning or scope of thepatent claims below. Covered embodiments of the invention are defined bythe claims, not this summary. This summary is a high-level overview ofvarious aspects of the invention and introduces some of the conceptsthat are further described in the Detailed Description section below.This summary is not intended to identify key or essential features ofthe claimed subject matter, nor is it intended to be used in isolationto determine the scope of the claimed subject matter. The subject mattershould be understood by reference to appropriate portions of the entirespecification, any or all drawings and each claim.

The present invention provides improved heat treatable aluminum alloyscontaining higher amounts of Mg than conventionally considered suitablefor heat treatment and can exhibit age hardening if solutionized incontinuous solution heat treatment lines. The improved aluminum alloysprovided herein can be produced as sheet alloys and can be more suitablefor recycling processes than conventional alloys. Some embodiments ofthe present invention are improved aluminum alloys suitable forfabricating automotive and other transportation machinery panels. Someother embodiments of the present invention are innovative new uses andapplications of the aluminum alloys, improved innovative processes formaking, fabricating or manufacturing aluminum alloys, processes forfabricating aluminum alloy forms, objects and parts, such as stampedsheet forms, the panels for transportation machinery. Aluminum alloyobjects, parts and forms that are fabricated from the improved aluminumalloys and/or according to the innovative processes provided herein arealso provided among the embodiments of the present invention.

One embodiment of the present invention provided herein is an aluminumalloy comprising ≧1.5% Mg by weight produced by a process comprisingheat treatment. The heat treatment process can comprise T4 temper. Thealuminum alloy can further comprise 0.2 to 0.4% Si by weight. Thealuminum alloy can undergo age hardening. The aluminum alloy can be asheet aluminum alloy. Another embodiment of the present inventionprovided herein is a stamped sheet form fabricated from the above sheetaluminum alloy. The stamped sheet form can be an automotive panel. Oneembodiment of the present invention provided herein is a process forfabricating a sheet aluminum alloy comprising ≧1.5% Mg and 0.2 to 0.4%Si by weight, comprising heat treatment. The process can comprise T4temper. The resulting sheet aluminum alloy can exhibit age hardening.One more embodiment of the present invention described herein is aprocess for fabricating a stamped sheet form, comprising stamping theabove sheet aluminum alloys. The stamped sheet form can be an automotivepanel.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a schematic diagram illustrating process steps used forproducing sheet aluminum alloys.

FIG. 2 is a schematic illustration of various sheet stampings used inautomobile production.

FIG. 3 is a bar graph showing DIN tensile properties of an alloy in Otemper and paint bake.

FIG. 4 is a bar graph showing tensile properties of an alloy in the T4,2% stretch, and 2% stretch followed by 20 min at 185° C.

FIG. 5 is a bar graph showing tensile properties of an alloy in the T4temper and after paint bake simulation (60 min at 180° C.).

FIG. 6 is a line plot illustrating age hardening of AA5251-T4 alloy.

DETAILED DESCRIPTION

In this description, reference is made to alloys identified by AAnumbers and other related designations, such as “series.” For anunderstanding of the number designation system most commonly used innaming and identifying aluminum and its alloys, see “International AlloyDesignations and Chemical Composition Limits for Wrought Aluminum andWrought Aluminum Alloys,” published by The Aluminum Association. 6xxxseries aluminum alloys, such as AA6111, AA6016 and AA6022, are typicallyused for producing automotive outer skin panels.

In general terms, 6xxx series alloys contain relatively high levels ofSi and low levels of Mg, are heat treatable, and exhibit age hardening,which confers on these alloys the strength parameters suitable forfabrication of the outer panels for transportation machinery, such asautomobiles. 5xxx series aluminum alloys in O temper, such as AA5182-Oor AA5754-O, are often preferred for inner panel fabrication inautomotive and related industries due to their formability properties.5xxx series aluminum alloys have very little tolerance to retain Si insolid solution. If Si is added to 5xxx series aluminum alloys, it tendsto combine with Mg to form coarse Mg₂Si particles during casting. Theseparticles are difficult to solutionize to produce super saturated solidsolution of Mg and Si during solutionizing and fast cooling on thecontinuous annealing lines. For this reason, 5xxx series aluminum alloyscontain relatively low Si levels and relatively high Mg levels, and areconsidered to be non-heat treatable due to their high Mg content. Thepresence of coarse Mg₂Si is potentially detrimental to formability.

Currently, 6xxx and 5xxx aluminum alloys cannot be easily combined andrecycled for fabrication of automotive and related panels, because theresulting recycled aluminum alloys may contain undesirably high levelsof both Si (as compared to 5xxx series aluminum alloys) and Mg (ascompared to 6xxx series alloys), and thus be neither suitable for heattreatment, due to high Mg levels, nor possess the formability of 5xxxseries alloys, due to a combination of relatively high Si and Mg levels.In addition, the presence of other metals, such as Cu, Mn, Fe or Zn, orcombinations thereof, present in the alloys recycled from thecombination of 5xxx and 6xxx alloys can lead to undesirable propertiesof the recycled aluminum alloys. For example, an undesirable combinationof properties can leave a recycled aluminum alloy unsuitable forfabrication of either inner or outer panels for transportationmachinery.

The inventors discovered that alloys that contain relatively high levelsof Mg, such as ≧1.5% Mg, are heat treatable and exhibit age hardening,if appropriate amounts of Si and/or Cu are present in such alloy. Thisproperty makes aluminum alloys with relatively high magnesium content,as compared to traditional 6xxx alloys, unexpectedly and advantageouslysuitable for applications where age hardening is desirable. Forinstance, the inventors discovered that some aluminum alloys containinghigher amounts of Mg than conventionally considered suitable for heattreatment, but lower amounts of Mg and higher amounts of Si incomparison to 5xxx series aluminum alloys traditionally used for innerautomotive panel fabrication, such as AA5754 or AA5182 alloys, canexhibit age hardening if solutionized in continuous solution heattreatment lines.

The inventors' discoveries are embodied in the improved aluminum alloysdescribed herein. The improved aluminum alloys described herein can beproduced as sheets, in which case they can be referred to as “sheetaluminum alloys,” “aluminum sheets,” “sheet alloys” or by other relatedterms, in singular or plural. The term “aluminum alloy” and similarterms used herein are broader in scope than “sheet aluminum alloy” andsimilar terms. In other words, sheet aluminum alloys are a subset ofaluminum alloys. Sheet aluminum alloys can possess the same or similarcomposition but, in some instances, different properties than the samealloy not in a sheet form. Some of these properties may be conferred bythe manufacturing or fabrication processes used in the production ofsheet aluminum alloys.

The improved aluminum alloys that embody applicants' discoveries exhibitage hardening similarly to 6xxx series alloys. They can also exhibitformability properties similar to those of 5xxx series aluminum alloys.The improved aluminum alloys are heat treatable. The improved aluminumalloys can be suitable for fabricating automotive and othertransportation machinery panels, and, more generally, in theapplications where high-Mg 5xxx series alloys are traditionally used.Increased content of Si and/or Cu in the improved aluminum alloysaccording to some embodiments of the present invention is beneficial inthe applications where age hardening is desirable, because Si and/or Cuare capable of conferring hardening on solutionized alloys due toprecipitation of Mg₂Si and Al₂CuMg particles during natural orartificial ageing. In addition to Si and/or Cu, some other elements canbe present in the improved aluminum alloys described herein in higheramounts than in some 5xxx series aluminum alloys conventionally used forfabrication of automotive panels. The presence of such elements canconfer advantageous properties on the improved aluminum alloys describedherein. For example, increased levels of Mn may promote formation ofdispersoids, which can help to disperse slip, thus improvingformability. The inventors also discovered that improved aluminum alloysdescribed herein are more suitable for recycling processes thanconventional alloys, because the improved aluminum alloys are tolerantto relatively higher amounts of Si, Cu, Fe or Mn, as compared to 5xxxseries aluminum alloys conventionally used for automotive panelmanufacturing, such as AA5754 and AA5182 alloys. Accordingly, improvedrecycling processes embody some of the inventors' discoveries.

In addition to the improved aluminum alloys, the inventors' discoveriesare embodied in innovative new uses and applications of the aluminumalloys, in improved innovative processes for making, fabricating ormanufacturing aluminum alloys, in the processes for fabricating aluminumalloy forms, objects and parts, such as stamped sheet forms, the panelsfor transportation machinery. Aluminum alloy objects, parts and formsthat are fabricated from the improved aluminum alloys and/or accordingto the innovative processes described herein also embody the inventors'discoveries.

Alloys

The improved aluminum alloys according to the embodiments of the presentinvention differ from the conventional alloys used in automotiveapplications in that they contain higher levels of one or more of Si,Cu, Fe, Mn, or Zn and lower levels of Mg, than at least some of 5xxxseries alloys and/or higher levels of Mg than at least some 6xxx seriesalloys The composition of the improved aluminum alloys is illustrated inTable 1, below. The content of the listed element can fall within theranges delimited by a lower range limit and an upper range limit shownin Table 1. A lower range limit can be delineated by expressions “equalto or more than” (≧ sign) or “more than” (> sign), or other relatedsigns and expression, such as “from . . . ,” “higher than” etc. An upperrange limits can be delineated by expressions “equal to or less than” (≦sign), “less than” (< sign) or other related signs and expressions, suchas “to,” “less than,” etc. Other types of expressions can also be usedto delineate the ranges, such as “between,” “in the range of,” etc. Whena range is delineated by only the upper range limit, it is to beunderstood that, in some examples, an element in question may not bepresent, may not be present in detectable quantities, or may be presentin such low quantities that they are conventionally not recognized asmeaningful in the field of aluminum alloys. It is also to be understoodthat some other additives and/or elements can be present in the aluminumalloys described herein, which are not necessarily listed in the tablesbelow.

TABLE 1 Composition of improved aluminum alloys (element content in wt%) Examples of Examples of Ele- lower range upper range Range examplesment limit limit Range 1 Range 2 Range 3 Mg 1.5; 1.55; 1.8; 1.85; 1.91.6 to 2   1.65 to 1.9 1.6; 1.61; 1.62; 1.63; 1.64; 1.65 Cu 0.3; 0.35;0.4; ≦0.8 ≦0.5 ≦0.3  0.45; 0.5; 0.55; 0.6; 0.65; 0.7; 0.75; 0.8 Fe 0.35;0.4; 0.45; ≦0.5 ≦0.4 ≦0.35 0.5 Mn 0.4 ≦0.4 Si 0.2 0.4 0.2 to 0.4 Zn0.25; 0.3; 0.35; ≦0.5 ≦0.3 ≦0.25 0.4; 0.45; 0.5 Cr  ≦0.25  ≦0.20 ≦0.15

TABLE 2 Exemplary composition of conventional 5xxx series alloys used inautomotive applications (element content is expressed in wt %) ELEMENTAA5182 AA5754 Mg 4 to 5 2.6 to 3.6 Cu ≦0.15 ≦0.10 Fe ≦0.35 ≦0.40 Mn 0.2to 0.5 ≦0.50 Cr ≦0.10 ≦0.30 Si ≦0.20 ≦0.40 Zn ≦0.20 ≦0.20

Properties and Advantages

Improved aluminum alloys described herein, including sheet aluminumalloys, possess one or more properties that make them suitable for theuse in automotive applications, such as fabrication of automotive panelsor, more generally, panels for various types of transportationmachinery, or, even more generally, stamped sheet forms. Some of theseproperties are formability, yield strength and age hardening. Improvedaluminum alloys also possess advantageous recycling compatibility with6xxx series aluminum alloys, such as AA6111, AA6022 or AA6016. Theexpression “recycling compatibility” and related terms are used hereinto describe a notion that improved aluminum alloys according to someembodiments of the present invention can be combined with 6xxx seriesalloys (and, optionally, other alloys or elements) during metallurgicalprocesses to fabricate commercially and technologically useful aluminumalloys, which can be characterized as “recycled.”

Formability and Paint Bake Response

Formability properties of the aluminum alloys described herein can beinfluenced by a number of variables. Formability properties include, butare not limited to, deep drawability and stretchability. One variableaffecting formability properties is the composition of an aluminumalloy. For example, formability, including castability, is influenced bythe amounts of Mg, Cu and Si in an aluminum alloy. High combined amountsof Mg, Si and/or Cu generally make it more difficult to cast and hotroll an aluminum alloy. Accordingly, the content of one or more of theseelements can be varied to arrive at the desired formability properties.Other variables that can affect formability are fabrication processvariations and conditions, such as, but not limited to, aluminum sheetprocessing steps and conditions, surface texturing process steps andconditions and lubrication process steps and conditions. One or more ofthe above variables can be adjusted to achieve desired formabilityproperties. Another important property that can be varied by one or moreof the variables discussed above is paint bake response of an aluminumalloy, which refers to change in strength during the paint cure process.Paint bake response is usually tested in the laboratory by ageing thedeformed or nondeformed material in the T4 temper at elevatedtemperature. The exact simulation conditions determine the paint bakeresponse vary from one car company to the other. For example, the paintbake response can be defined as change in strength by ageing an aluminumalloy at 180° C.

Strength

The improved aluminum alloys according to the embodiments of the presentinvention can exhibit 80 to 160 MPa yield strength (YS), which can besimilar or equivalent to that of AA5754 or AA5182 in a typical finishedand painted part required for automotive application. In someembodiments, strength of an improved aluminum alloy is influenced byincreasing an amount of Cu in the aluminum alloy, as compared to Cucontent of the alloys conventionally used for fabrication of panels forautomobiles and other transportation machinery.

Hardness

Certain embodiments of the improved aluminum alloys described herein areheat treatable and exhibit age-related hardening, while exhibitingformability comparable to typical 5xxx aluminum alloys conventionallyused in automotive applications. 5xxx aluminum alloys were previouslynot known to be heat treatable or exhibit age related hardening uponheat treatment. Improved aluminum alloys according to some embodimentsof the present invention contain higher levels of Mg than the aluminumalloys conventionally recognized as heat treatable. Some examples of theimproved aluminum alloys of the present invention contain ≧1.5% of Mgand are heat treatable. The presence of appropriate amounts of Si and/orCu confers heat treatability and age hardening properties on an improvedaluminum alloy containing ≧1.5% of Mg. This allows some improvedaluminum alloys according to the embodiments of the present invention toachieve an unexpectedly advantageous combination of formability(conferred by higher Mg levels than those conventionally present inheat-treatable alloys) and age hardening upon heat treatment such as T4temper (conferred by higher Si levels than those conventionally presentin 5xxx series alloys).

In comparison to some of the 5xxx aluminum alloys, such as thoseconventionally employed for manufacturing of inner automotive panels, insome embodiments improved aluminum alloys of the present inventioncontain reduced amount of Mg. Reduced levels of Mg can result in lowercost of the improved aluminum alloys described herein, as well as in thelower costs of the forms the objects manufactured from such alloys,since less Mg is required for production. Reduced levels of Mg in theimproved aluminum alloys described herein can also result in improvedsolubility of Si in aluminum during solutionizing, which advantageouslyaffects the properties of the alloys. Both Si and Cu are capable ofimproving hardening of solutionized the improved aluminum alloysdescribed herein due to precipitation of Mg₂Si and Al₂CuMg or Q(AlMgSiCu) containing particles during ageing.

Recyclability

The improved aluminum alloys of this invention possess a tolerance forhigher amounts of Si than conventional 5xxx series alloys used formanufacturing of automotive panels. This higher tolerance for Si and/orthe ability of the improved aluminum alloys described herein to exhibitpaint bake response makes them suitable and compatible with 6xxx alloysfor recycling.

In summary, the improved aluminum alloys of the present invention havean advantageous combination of properties that allows these improvedalloys to be used in place of conventional high-Mg aluminum alloys forvarious applications. The improved aluminum alloys described herein canexpand the range of alloys available for a variety of applications, oneof which is manufacturing of stamped sheet forms, such as panels forautomobiles and other transportation machinery, increase aluminumrecycling rates, lower the costs of aluminum alloy manufacturing, anddecrease the environmental impact of aluminum production

Fabrication Processes

The processes for making or fabricating the improved aluminum alloys arealso included within the scope of the present invention. Improvedaluminum described herein can be fabricated by the processes thatinclude at least some of the technological steps described below. Atleast some of these technological steps can confer advantageousproperties on the improved aluminum alloys. It is therefore important,in some cases, to include process steps when describing the improvedaluminum alloys. For example, one exemplary embodiment of an improvedaluminum alloy described herein is AA5251 alloy. Prior to the inventors'discovery, AA5251 alloy, which contains >1.5% Mg, was not known to besuitable for heat treatment, and to exhibit age hardening, when in theT4 temper. Accordingly, an exemplary embodiment of improved aluminumalloys described herein is AA5251 alloy in T4 temper, which can bereferred to as AA5251-T4.

The processes of making or fabricating the improved aluminum alloys caninvolve heat treating in order to alter the physical and/or chemicalproperties of the improved aluminum alloys. Heat treatments involve theuse of heating and/or chilling, of an aluminum alloy to achieve adesired result, such as hardening. An embodiment of the processesdescribed herein employs T4 or T4P temper, which involves solution heattreatment and natural aging of an aluminum alloy to a substantiallystable condition. T4P temper refers to special thermal heat treatmentincluded following solutionizing. This treatment can be implementedeither by controlled cooling from solutionizing temperature or bereheating to a temperature ranging from 50 to 110° C. within an hour ofsolutionizing. In some other embodiments, T6 and T8 tempers can also beused.

It is to be understood that descriptions and illustrations of theprocesses described herein are non-limiting. The process steps describedherein can be combined and modified in various ways and suitablyemployed for fabricating the improved aluminum alloys or forms andobjects from such alloys. Process steps and conditions that are notexplicitly described herein, yet commonly employed in the areas ofmetallurgy and aluminum processing and fabrication, can also beincorporated into the processes described herein.

One exemplary process is schematically illustrated in FIG. 1. It is tobe understood that one or more of the process steps illustrated in FIG.1 can be incorporated into the processes for making improved aluminumalloys.

Another example of a process that incorporates one or more steps thatcan be combined in various ways and suitably employed for fabricatingthe improved aluminum alloys is described in this paragraph. An improvedsheet aluminum alloy is produced from a direct chilled (DC) ingot.However, the hot rolling stock may also be produced from a continuouscast slab. The DC cast ingots are scalped to remove near surfacesegregation layer on both sides of the ingot and homogenized at atemperature between 500 and 575° C. for time periods between 1 to 48hours before being subjected to hot and cold rolling to the final gauge.Improved sheet aluminum alloy can also be subjected to special surfacetexturing, such as, but not limited to, electro discharge texturing, inorder to improve formability of the final sheet. The cold rolled stripis solutionized by heating at >3° C./s in a continuous annealing line toa temperature between 500 and 575° C., followed by fast cooling andnatural ageing to produce sheet in the T4 temper. Solution heattreatment can re-dissolve soluble particles, such as Mg₂Si or otherparticles back into the matrix, depending on the alloy composition. Fastquenching is used to produce a super saturated solid solution, in termsof both solutes and excess vacancies. The fast cooling from thesolutionizing temperature can be carried out in forced air, water mist,or combination of both water mist and forced air. Coiling is performedat a temperature between 50 to 110° C., followed by coil cooling at arate ≦10° C./hour. The coil can be reheated in the strip form to ensurethe coiling temperature between 50 to 110° C. It is possible to subjectthe solutionized sheet alloy to either acidic or alkaline cleaning,followed by pre-treatment with special chemicals and lubricants, oils orwaxes before coiling at a temperature between 50 and 110° C. The coilcan be blanked and used for stamping inner panels, such as thoseillustrated in FIG. 2.

Yet another example of a process that incorporates one or more stepsthat can be combined in various ways and suitably employed forfabricating the improved aluminum alloys is described in this paragraph.A direct chilled cast alloy ingot is homogenized above 500° C. for ≧2hours, hot rolled to an intermediate gauge with coiling temperaturebetween 280 to 400° C., cold rolled to the final gauge in one or morepasses with either mill or optimized finished texture and solutionizedin the strip form at temperatures above 480° C. in a continuousannealing line, fast cooled and coiled between 50° C. and 120° C. Thehot coiling step is optional and is used to improve the paint bakeresponse of the alloy. In some situations, the solutionized coil mayalso be cleaned, pretreated and lubricated prior to stamping.

The following discussion is included to illustrate the advantageousproperties that fabrication process steps can confer onto the improvedaluminum alloys described herein. Traditionally, AA5754 or AA5182 alloysare supplied for manufacturing of automotive panels in the soft Otemper, so that a part can be formed from these alloys and thensubjected to paint cure operation. AA5754 or AA5182 in O temper exhibitsoftening due to recovery during paint bake. The improved aluminumalloys according to some embodiments of the present invention are notsubject to such softening or are not subject to it to the same extent asAA5754 or AA5182 in O temper. The improved aluminum alloys describedherein can maintain strength closer to AA5754 and AA5182 after formingand paint cure. For example, the strength properties on the final partmanufactured from the improved aluminum alloys of the present inventioncan be similar or equivalent to AA5754 alloy.

Uses and Applications

Uses and applications of the improved aluminum alloys described hereinare included within the scope of the present invention, as are objects,forms, apparatuses and similar things fabricated from or comprising theimproved alloys described herein. The processes for fabricating,producing or manufacturing such objects, forms, apparatuses and similarthings are also included within the scope of the present invention. Forexample, some embodiments of the improved aluminum alloys describedherein are suitable for manufacturing of automotive panels. Variousautomotive panels, including inner and outer automotive panels, aretherefore included within the scope of the present invention. They aredescribed, for example, in U.S. Patent Publication No. 2010/0279143, andare also illustrated in FIG. 2.

It is to be understood, however, that the uses and applications of theimproved aluminum alloys and objects that are manufactured from suchalloys are not limited to automobile panels. Other objects can besuitably manufactured from the improved aluminum alloys describedherein. One example is the panels generally incorporated into varioustransportation vehicles and other moving machinery, which can be termed“transportation panels” or “machinery panels.” For instance, the panelsused for transport trucks can be advantageously manufactured from theimproved aluminum alloys described herein. Transport trucks withaluminum cabs are traditionally produced from AA5052 alloy. This alloyhas a tendency to exhibit stretch bands or yield point elongation duringforming, causing objectionable surface appearance. Improved aluminumalloys according to some embodiments of the present invention do notexhibit yield point elongation and can be used to advantageously replaceAA5052 alloy for manufacturing of panels used in transport trucks.

More generally, some embodiments of the improved aluminum alloysdescribed herein, in comparison to conventional 5xxx alloys, show lesstendency to display Liiders bands, also known as “slip bands” or“stretcher-strain marks,” which are localized bands of plasticdeformation in metals experiencing tensile stresses. Accordingly, theimproved aluminum alloys described herein can be advantageously employedin the manufacturing of parts or objects where Liiders bands areobjectionable, such as outer panels for automobiles and othertransportation vehicles and moving machinery.

Some embodiments of the alloys described herein are suitable for complexelectronic applications. One example of such application is aluminum TVframes. More generally, various sheet stamping, stamped sheet forms,stamped panels, or related objects fabricated from the improved aluminumalloys described herein are included within the scope of the embodimentsof the present invention.

The following examples will serve to further illustrate the presentinvention without, at the same time, however, constituting anylimitation thereof. On the contrary, it is to be clearly understood thatresort may be had to various embodiments, modifications and equivalentsthereof which, after reading the description herein, may suggestthemselves to those skilled in the art without departing from the spiritof the invention. During the studies described in the followingexamples, conventional procedures were followed, unless otherwisestated. Some of the procedures are described below for illustrativepurpose.

Example 1 Testing of Tensile Properties of AA5251 Alloy in O Temper

An aluminum ingot containing 1.85% Mg, 0.3% Fe, 0.28% Mn and 0.29% Siwas homogenized at 540° C. for >5 hours, hot rolled to 3.2 mm gauge,cold rolled to the final 1.3 mm gauge and batch annealed for 1 hour at340° C. to obtain O temper. The transverse tensile properties of theannealed sheets were determined using DIN specimens. FIG. 3 shows theDIN tensile properties of the alloy in both O and paint bake (5% stretchplus 20 min at 185° C.). The alloy exhibited 70 MPa yield strength (YS),164 MPa Ultimate Tensile Strength (UTS) and 23% total elongation in theO temper and showed no hardening after ageing for 20 min at 185° C. Thehigher YS in the paint bake temper (5% stretch plus 20 minutes at 185°C.) is the net result of work hardening due to stretching and recoverydue to ageing.

Example 2 The Effect of Solutionizing on the Tensile Properties ofAA5251 Aluminum Alloy

This example shows the effects of solutionizing on the tensileproperties of an aluminum alloy. An aluminum ingot containing 1.85% Mg,0.3% Fe, 0.28% Mn and 0.29% Si was homogenized at 540° C. for >5 hours,hot rolled to 3.2 mm gauge and cold rolled to the final 1.3 mm gauge.The cold rolled 1.3 mm gauge sheets were solutionized for 2 min at 560°C., cooled and immediately pre-aged for 8 h at 85° C. The transverseASTM properties of the solutionized alloy were determined after 24 hoursof natural ageing. FIG. 4 shows comparative tensile properties of thealloy in the T4 temper, 2% stretch and 2% stretch plus 20 min at 185° C.tempers. The aluminum alloy in the T4 temper was stronger in comparisonto its O temper counterpart, as illustrated by the comparison of FIGS. 3and 4. The aluminum alloy in T4 temper exhibited a significant increasein YS due to 2% stretch and after subjecting the stretch sample toageing at 185° C. for 20 min. The tensile properties of the aluminumalloy in the T4 temper were close to the conventional AA5754 alloy. Theyield strength of the aluminum alloy was close to the expected strengthof AA5182 or AA5754 alloy, after subjecting it to similar paint baketreatment.

Example 3 The Role of Cu Addition to an Alloy

An aluminum ingot containing 1.75% Mg, 0.78% Cu, 0.23% Fe, 0.11% Mn and0.38% Si was homogenized at 560° C. for >18 hours, then hot rolled andcold rolled to the final 1.6 mm gauge and solutionized in a continuousannealing line at 540° C., cooled and pre-aged. The transverse tensileproperties of the 1.6 mm gauge sheets were determined using ASTMspecimens.

FIG. 5 shows the tensile properties of the alloy in both T4 and paintbake (60 min at 180° C.). This alloy, which contains higher levels ofcopper than AA5251 alloy discussed in Examples 1 and 2, wassignificantly stronger in comparison to the AA5251 alloy. The alloytested in this example exhibited 143 MPa YS, 284 MPa UTS and 28% totalelongation in the T4 temper, and showed significant hardening afterageing for 60 min at 180° C. due to precipitation of CuMgAl₂ and Mg₂Siparticles.

Example 4 Comparative Testing of AA5754 in O Temper and AA5251 in O andT Tempers

The aluminum ingots of AA5754 and AA5251 alloy having the compositionshown in Table 3 were homogenized at 540° C. for >5 hours, hot rolledand cold rolled to the final 1 and 1.3 mm gauges, respectively, inseparate trials. Coils of AA5754 and AA5251 were solutionized on thecontinuous annealing line at 500 and 560° C., respectively.

The tensile test results from the trial coils are shown in Table 4. Itcan be seen that the yield strength and ultimate tensile strength of theconventional AA5754 sheet in O temper in the 0°, 45° and 90° withrespect to the rolling direction is close to 100 MPa and within the 219to 231 MPa range, respectively. AA5251 alloy in O temper exhibits lowervalues compared to the AA5754, except for the strain hardening exponent(n) value. AA5251 alloy in T temper exhibits significant improvement instrength properties, such as yield strength and ultimate tensilestrength, compared to AA5251 O temper alloy. In terms of strength,AA5251 T temper alloy falls between AA5754 and AA5251-O temper. AA5251 Ttemper alloy exhibits paint bake response typically not observed in theAA5251 and AA5754-O temper alloys. The detected improvements in AA5251 Ttemper alloy offer a possibility of using it as a substitute for AA5754and possibly AA5182 alloys. Marginally inferior forming characteristicsof AA5251 T temper alloy, indicated by lower elongation, UTS and nvalues can be compensated by variety of techniques including optimizingalloy and process composition, using preferred sheet surface texture, orchoice of lubricant during forming.

TABLE 3 Aluminum alloy composition Composition wt % Alloy Cu Fe Mg Mn SiCr Ti AA5754 0.02 0.20 3.10 0.22 0.06 0.05 AA5251 0.01 0.30 1.83 0.300.29 0.03 0.01

TABLE 4 Comparative testing results of AA5754 in O temper and AA5251 inO and T tempers Yield Tensile Gauge Strength Strength Total Temper Dirmm ksi MPa ksi MPa Elongation % n R CASH Annealed AA5754 Coil#2271809 O0 1.0 14.6 101 33.4 230 26 0.30 0.87 45 14.5 100 32.3 223 26 0.31 0.5790 14.4 99 31.9 220 24 0.31 0.64 Batch Annealed AA5251 Coil#L55203R1 O 01.3 9.7 67 24.9 172 23 0.34 0.67 45 9.6 66 24.2 166 26 0.32 0.59 90 9.666 23.7 163 21 0.32 0.55 CASH Solutionizd AA5251 Coil#L55203R2 T4 0 14.197 28.4 196 26 0.26 0.81 45 13.9 96 27.8 192 26 0.26 0.55 90 13.8 9527.4 189 24 0.26 0.61 2% + 20 min @ 0 20.1 139 31.5 217 21 0.21 0.83185° C. 45 20.0 138 31.0 214 20 0.21 0.52 90 19.8 137 30.5 210 20 0.210.60

Example 5 Age Hardening of AA5251 T4 Temper Alloy at 185° C.

Age hardening studies of AA5251 T4 temper alloy were performed byplacing tensile samples of the alloy in a furnace set at 180° C. Thesamples were taken out of the furnace after different ageing times. FIG.6 shows the ageing hardening behavior of the alloy at 180° C. The alloyexhibited about 70% and 20% increase in YS and UTS, respectively, afterabout 8 h of ageing. The results illustrated in FIG. 6 support aconclusion that the alloy underwent age hardening.

All patents, publications and abstracts cited above are incorporatedherein by reference in their entirety. Different arrangements andcombinations of the elements and the features described herein arepossible. Similarly, some features and subcombinations are useful andmay be employed without reference to other features and subcombinations.Various embodiments of the invention have been described in fulfillmentof the various objectives of the invention. It should be recognized thatthese embodiments are merely illustrative of the principles of thepresent invention. Numerous modifications and adaptations thereof willbe readily apparent to those skilled in the art without departing fromthe spirit and scope of the present invention.

1. An aluminum alloy comprising ≧1.5% Mg by weight produced by a processcomprising heat treatment.
 2. The aluminum alloy of claim 1, wherein theprocess comprises T4 temper.
 3. The aluminum alloy of claim 1, whereinthe aluminum alloy further comprises 0.2 to 0.4% Si by weight.
 4. Thealuminum alloy of claim 1, wherein the aluminum alloy exhibits agehardening.
 5. The aluminum alloy of claim 1, wherein the aluminum alloyis a sheet aluminum alloy.
 6. A stamped sheet form fabricated from thesheet aluminum alloy of claim
 5. 7. The stamped sheet form of claim 6,wherein the stamped sheet form is an automotive panel.
 8. A process forfabricating a sheet aluminum alloy comprising ≧1.5% Mg and 0.2 to 0.4%Si by weight, comprising heat treatment.
 9. The process of claim 8,wherein the process comprises T4 temper.
 10. The process of claim 8,wherein the sheet aluminum alloy exhibits age hardening.
 11. A processfor fabricating a stamped sheet form comprising stamping the sheetaluminum alloy of claim
 5. 12. The process of claim 11, wherein thestamped sheet form is an automotive panel.